Apart from being based on their good mechanical and thermal properties, the multifarious application of sintered components is also based on the great variety of further advantageous properties such as, for example, the good chemical and corrosion resistance of ceramic materials, which also enables good compatibility with living tissue or an exactly adaptable resorbability of bone substitute materials. Other applications of ceramic materials result from their electrical properties such as their good insulation and specific ferro- or dielectric properties.
The extensive range of adjustable microstructures and/or combinations of properties, the achievable complex component geometries and the good possibilities for combination with other materials have had the effect that ceramic materials often enjoy a key role in the development of new products. Alongside the classic applications of ceramic components, for example as a heating element, insulator, ball bearing and as a seal, it is also the case that medical applications, for example as a substitute for bones, teeth and joints, as well as applications in microsystem technology are increasingly occurring, ceramic materials enabling the construction of more complex multilayer wiring substrates with multifarious integrated functionalities, in the last mentioned applications. Such components are, for example, increasingly being used in the automobile industry and in information and medical technologies. In particular, modern innovative applications of ceramic materials do, however, place very high demands on their reliability—that is to say, the variation of the desired properties—as well as on the precision of the component geometry.
In the case of sintered components, the shape and size as well as the achievable material properties and their variation are inextricably bound up with the processes for their production. It is therefore decisive for the quality of the end product for these processes to be controlled as precisely as possible. This control is therefore a substantial precondition for the market introduction of ceramic components, and influences the present day technological progress in a decisive way.
Particularly in the case of green bodies of inhomogeneous porosity, complex geometry, or with a heterogeneous material construction, it is possible for local differences to occur during compaction. The sintered component then differs from the green body not only with reference to size, but also as regards its shape. In order to correct such effects, increasing use is being made nowadays of 3D sintering simulation tools, for example on the basis of finite element methods, in order to predict the 3D sintering behavior. The findings obtained can then be used, for example, to adjust the shape of the green body such that the sintered component assumes the desired shape.
In addition, in the case of green bodies of inhomogeneous porosity, complex geometry or with a heterogeneous material construction, upon compaction it is also possible for temporary local differences to result, which need not be recognizable from the shape of the fired component. Thus, for example, the different shrinkage during sintering of a multiply film composite can result in temporary distortions. Such temporary warpings during sintering, or else just the formation of stresses in the sintered body can later cause microstructural inhomogeneities such as, for example, the formation of faults in the fired component, and thus lead to breakage or to some other type of failure of the component.
The increasing demands being placed on the precision and reliability of sintered components are therefore urgently necessitating novel methods for controlling the sintering process in a fashion integrated therewith and/or in situ, with such methods also being capable of detecting very slight and complex shape changes as well as local temperature differences of the surface during the sintering process in a precise way, that is to say down to the μm region. This information can then be used to optimize the production of shaped bodies, to the assessment and development of 3D sintering simulation tools or to the control of the sintering process itself.
As a rule, however, industrial sintering processes have so far proceeded without in-situ control of the shrinkage, this being done subsequently. Only individual measured variables such as temperature are measured in the furnace interior, evaluated or used as controlled variable. At present, even the assessment and optimization of 3D simulation tools, which can, after all, predict the entire sintering process, is still based on the measurable properties of the finished or partially sintered component.
Concerning in-situ control of shrinkage, the application of optical silhouette methods by means of a heating microscope and image evaluation is known in the research—see, for example, F. Raether, R. Springer, S. Beyer: Optical dilatometry for the control of microstructure development during sintering; Mat Res Innovat (2001) 4, p. 245-250. FIG. 5a shows the principle of the silhouette method of the prior art. A sample 104 with, for example, a circular cross section is irradiated from one side by a light source 102a. In the example, the sample 104 has a central depression on the surface facing the light source 102a. The light source 102a irradiates the sample 104 from one side. On the side of the sample 104 opposite thereto, a CCD camera is arranged as detector 102b. It receives the light from the light source 104 that passes the sample. Wherever the sample 104 blocks the beam path between the light source 102a and detector 102b, a shadow of the sample 104 is produced on the CCD camera 102b. As shown in FIG. 5b, the basic shape of the sample 104 is obtained as a silhouette, as a result. In the example, this is merely an undifferentiated circle without information relating to the depression of the surface of the sample 104. The silhouette method can be used to carry out only two-dimensional shrinkage measurements on small, simple and, in particular, convexly curved samples, for example a 30 mm high cylinder with a diameter of 11 mm, where the silhouette can supply sufficient information relating to the sample shape. Since the silhouette constitutes the projection of the light-dark boundary of the object onto the observation plane, it includes no information relating to the distance of this line, and thus relating to the actual spatial profile and the length of said boundary. Only in the case of regularly shaped objects and accurately defined imaging conditions such as, for example, a sphere, is it possible to reach a conclusion as to the actual surface profile of the object and its spatial shape indirectly from the silhouette. The term “indirectly” indicates here that with this method it is necessary to make assumptions relating to the shape of the object. The description of irregular shape changes, for example the complex warpings of large-format sintered films, for example in the 8″×8″ format, is impossible with the silhouette method.
Non-contact optical methods for in-situ detection of the three-dimensional geometry of bodies even at high temperatures are known in conjunction with other technical tasks not included in sintering technology. These are based on the method of triangulation. It is mostly a laser that is used here as light source. An electronic image converter, mostly a CCD camera, then records the scattered light. FIG. 6a shows an example of the principle of the measurement of the scattered or backscattered light. Once again, a circular sample 104 with a depression in its surface as in FIG. 5a is irradiated from one side with a light source 102a. By contrast with the silhouette method, the light source 102a and detector 102b are now arranged on the same side of the sample 104 at a certain distance and/or angle from one another. Light strikes the sample 104, is backscattered and passes into the detector 102b. The surface profile of the sample 104 can now be generated from the intensity of the detected (back) scattered light. This is shown in FIG. 6b and includes information relating to the differentiated structure of the surface of the sample 104 with the depression.
U.S. Pat. No. 4,588,297 A describes a method as to how it is possible by means of triangulation to determine the filling level of the bulk material fed for the melt in a blast furnace. The filling level is determined in this case by means of pulsing a laser beam on the surface, while the angle of the emitter to the surface is varied incrementally along a prescribed path. The scattered light is then measured by means of a detector with a small aperture angle. The detector and emitter are arranged in this case at a fixed spacing. Points along the profile are determined from the emission and reception angles and the known distance between the emitter and detector. To this end, the laser and light receiver are located outside the furnace in front of the observation windows. Since the laser and receiver do not form a rigidly connected unit, but are fitted separately on the furnace, and the incidence angle of the laser beam is adjusted in small angle steps during measurement in order to determine a profile, the calibration of this arrangement is difficult and, moreover, very susceptible to faults, for example owing simply to the thermal expansion of mechanical components of the sensor fastening during heating. The measuring accuracy of the arrangement is therefore very limited.
JP 2001-099615 describes a water-cooled triangulation sensor that operates with green laser light and is arranged directly in a high temperature furnace. The measurement of the surface of poured-in waste incineration material is known as one application. In order to be able to measure the visible surface of an object, adjustable parameters in the case of this sensor are the inclination of the laser beam and the inclination of the optical axis of the optical receiver in relation to a fixed coordinate axis. From these variable operating angles, the other fixed geometric optical parameters of the arrangement and the image coordinates of the image of the laser spot in the CCD camera, the 3D coordinates of the point emitted by the laser onto the object surface are determined within an evaluation unit for each angular setting. However, there are strict physical resolution limits which, given a discrete camera chip, are related to the distance between the measurement object and camera and the angle between the incident laser beam and the optical axis of the camera. The continuous angular tracking inherent in this method entails different resolutions and is therefore unsuitable for high accuracies in the μm region. Furthermore, the large distance between the surface to be measured and the detector has a disadvantageous effect on the accuracy in the case of this arrangement and also in the case of the first-mentioned measurement in a blast furnace.
At room temperature, arrangements are known in which the measurement object is rotated at the same time as the optical sensor is displaced linearly over the measurement object. Thus, U.S. Pat. No. 5,270,560 A describes a method that can be used to measure semiconductor wafers with a laser sensor. DE 195 04 126 A1 describes a method for measuring three-dimensional objects by optical triangulation, which can be used in shoe last measurement, in mold construction and in orthopedic, technical dental and archaeological applications.
However, there is to date no possibility for the direct and sufficiently accurate quantitative detection of two- or three-dimensional surface profiles of the object and changes relevant thereto during the sintering process at high temperatures. There is not as yet any method for the monitoring and control of the sintering of ceramic and powder metallurgical components in a high temperature furnace that includes a direct two- or three-dimensional shape recognition and shape analysis during the heating process.